The transcription factor FUSCA3 (FUS3) acts as a major regulator of seed maturation in Arabidopsis. FUS3 is phosphorylated by the SnRK1 catalytic subunit AKIN10/SnRK1α1, which belongs to a conserved eukaryotic kinase complex involved in energy homeostasis (Tsai and Gazzarrini, 2012; Plant J.). Here we show that AKIN10 and FUS3 share overlapping expression patterns during embryogenesis, and that FUS3 is phosphorylated by AKIN10 in embryo cell extracts. To understand the role of FUS3 phosphorylation, we generated fus3-3 plants carrying FUS3 phosphorylation-null (FUS3S>A) and -mimic (FUS3S>D) variants. While FUS3S>A and FUS3S>D rescued all the fus3-3 seed maturation defects, FUS3S>A showed reduced transcriptional activity and enhanced fus3-3 previously uncharacterized phenotypes. FUS3S>A embryos displayed increased seed abortion due to maternal FUS3S>A and delayed embryo development, which resulted in a strong decrease of seed yield (~50%), suggesting phosphorylation of FUS3 plays an important role during reproductive development and early embryogenesis. Accordingly, the akin10 and akin11 mutants displayed a frequency of seed abortion similar to fus3-3. FUS3S>D delayed flowering similarly to ML1:FUS3 overexpression. However, no FUS3 expression was detected during vegetative development, suggesting FUS3 phosphorylation may regulate the embryonic expression of genes involved in flowering time. When plants were grown at elevated temperature, most FUS3S>A phenotypes were exaggerated and next generation seedlings overall grew poorly. Collectively, these results suggest that FUS3 phosphorylation by SnRK1 is required for embryo development and integration of environmental cues to ensure the survival of the next generation (Chan et al., 2017; J. Exp. Bot., accepted). FUS3 and AKIN10 localizations in reproductive organs, as well as genetic interaction between fus3-3, akin10/11 and FUS3 phosphomutantswill be discussed.